1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal panel and a method of manufacturing the same. The present invention relates also to a display device employing the ferroelectric liquid crystal panel.
2. Description of the Prior Art
It is well known that ferroelectric liquid crystals provide a high-speed response in microseconds, because liquid crystal molecules thereof have spontaneous polarization. When the ferroelectric liquid crystals are made thin, they show a memory effect. Because of this, even if the number of display lines is increased, the ferroelectric liquid crystals provide a high contrast display. Furthermore, the ferroelectric liquid crystals are very wide in viewing angle. These superior properties can be realized with a simple matrix panel, which can be readily manufactured at a low cost (see U.S. Pat. No. 4,367,924).
FIG. 1 depicts a molecular orientation of a ferroelectric liquid crystal at the time a ferroelectric liquid crystal panel was made thin. It was confirmed by X-ray diffraction that liquid crystal molecules 20 were aligned parallel to upper and lower substrates by alignment layers formed on respective substrates, and liquid crystal layers 21 were bent between the substrates, as shown in FIG. 1.
Although chiral smectic liquid crystals are generally used as the ferroelectric liquid crystals, the smectic phase has a higher-order orientation than the nematic phase, and smectic liquid crystal molecules tend to orient by forming a layer structure. Due to the higher-order orientation, the smectic liquid crystals provide a high-speed response. When an easy-to-align low molecular ferroelectric liquid crystal panel is subjected to deformation, for example, by a mechanical shock, the molecular orientation and the layer structure thereof fall into disorder, and the memory effect is lost. As a result, no display can be presented on the ferroelectric liquid crystal panel. The disorder of orientation cannot be restored until the ferroelectric liquid crystal experiences processes wherein the temperature thereof is raised to a temperature of the nematic phase or isotropic phase and is subsequently gradually reduced.
Because the layer structure tends to fall into disorder if a cell warps, U.S. Pat. No. 4,674,839 discloses the use of a protection means operable to avoid direct transmission of an external force to the cell to thereby to avoid the possibility of the disorder in the layer structure. In order to prevent the external force from being applied to the cell by the use of the protection means, the cell must be kept out of contact with the protection means. To this end, it is necessary to make the protection means thick and to keep the cell a suitable distance away therefrom. As a result, a display unit becomes considerably thick, and the advantage of the liquid crystal panel is lost.
In U.S. Pat. No. 4,674,839, a flexible substrate is employed as one of substrates so that a liquid crystal layer may be made thin and uniform in thickness. This prior art document teaches that the layer structure is liable to be disordered by warping of the flexible substrate, which may be caused by an impact or the like.
U.S. Pat. No. 4,674,839 teaches a preferred structure wherein the flexible substrate is placed on the display side and a transparent protection means is spaced therefrom. In this structure, another substrate placed on a side opposite to the display side and the protection means are both made thick to restrain warping the cell. As a result, a display unit becomes thick.
Japanese Laid-open Patent Publication No. 3-203773 discloses a liquid crystal panel having an air damper. The air damper is formed by a closed space defined by a cell and a support member. The provision of the air damper is based on the fact that the protection means is not so effective against a mechanical shock caused by inertia, which would be produced, for example, by spontaneous falling of the cell. As is the case with the protection means, the provision of the air damper also makes a display unit thick. In addition, the structure becomes complicated, thus increasing the cost of manufacture.
Japanese Laid-open Patent Publication No. 64-33524 discloses a method of bonding upper and lower substrates via spacers to rigidify a cell itself. Although this method can enhance resistance to impact shock to some extent, it is necessary to inject a liquid crystal into a space defined between the upper and lower substrates after both the substrates have been bonded. Because ferroelectric liquid crystals are generally very high in viscosity, and the spacing between the substrates is extremely narrow, it is very difficult to uniformly inject a liquid crystal into a large cell.
According to experiments conducted by the inventors of the instant application, when two generally available panels each having a thickness of 1.1 mm or 0.7 mm were bonded, the strength thereof was approximately three times greater than the case where these panels were not bonded. However, when the panel surface was depressed somewhat strongly by a finger, an uneven display occurred.
U.S. Pat. No. 4,682,858 teaches making a liquid crystal layer thin and unifying the thickness thereof using a flexible substrate as one of two substrates. In general, the thickness of the liquid crystal layer showing the memory effect ranges from 1 .mu.m to 2 .mu.m. This range in thickness is appropriate to obtain a desired memory effect and also to approximate the color produced by birefringence effect to water-whiteness. According to this disclosure, because the liquid crystal layer is thin and has a small tolerance of, for example, 0.1 .mu.m in thickness, as compared with conventional TN (Twisted Nematic) type liquid crystals, if a cell is defined by generally available glass plates, it is impossible to uniformly form the thin liquid crystal layer therein. Accordingly, this disclosure teaches placing the flexible substrate in accord with undulation of another thin substrate made of glass or plastic film to prevent unevenness in thickness of the liquid crystal layer. According to this disclosure, after the two substrates have been bonded via spacers, the cell is evacuated to allow the liquid crystal to be injected thereinto through an injection hole. During the injection, however, because the cell is subjected to a pressure difference of approximately 1 atm between the inside and the outside thereof, the spacers cut into the plastic film, thus considerably narrowing the distance between the two substrates. As a result, it is extremely difficult to inject the liquid crystal into the cell.
According to this disclosure, supposing that the flexible substrate is an elastic plate which is carried by two spaced supports and which is subjected to deformation under the influence of an external force, the deformation of the flexible substrate is expressed by a formula. Based on this formula, the distance between the spacers is determined. In the case of a plastic substrate, however, the spacers are likely to cut thereinto. In particular, of generally available spherical spacers, hard ones cut into the substrate with ease. Accordingly, the aforementioned formula indicative of elastic deformation is not applicable to such a case. Even in the case of a glass substrate, because the spacers are likely to cut thereinto according to the relationship between the hardness thereof and that of electrodes, insulating layers, and alignment layers on the substrate, the aforementioned formula is not applicable to this case also. Furthermore, the provision of a reduced pressure space inside the cell reduces the volume of the liquid crystal at low temperatures, thereby enlarging a vacant internal space. This space occasionally appears on the display or slightly remains even if the temperature of the cell is returned to normal temperature. In some cases, a defect of molecular orientation would occur.
Although glass is generally used for substrates of a liquid crystal panel, Japanese Laid-open Patent Publication Nos. 56-155920 and 64-33524 disclose the use of plastic films for the substrates to make them thin. According to these disclosures, transparent electrodes of, for example, indium tin oxide are initially mounted on a transparent plastic resin film of, for example, polyether sulfon, polycarbonate, polyethylene terephthalate, or the like. Then, an alignment layer is coated on the plastic resin film, and liquid crystal is appropriately oriented through a rubbing treatment. In Japanese Laid-open Patent Publication No. 64-33524, a cell is fabricated by bonding upper and lower substrates to each other via spacers coated with an adhesive. Thereafter, a ferroelectric liquid crystal is injected into the cell.